In general, an air conditioner for a vehicle includes a cooling system for cooling the interior of the vehicle and a heating system for heating the interior of the vehicle.
At an evaporator side of a refrigerant cycle, the cooling system converts air into cold air by exchanging heat between the air passing outside an evaporator and a refrigerant flowing inside the evaporator so as to cool the interior of the vehicle. At a heater core side of a coolant cycle, the heating system converts air into warm air by exchanging heat between the air passing outside the heater core and coolant flowing inside the heater core so as to heat the interior of the vehicle.
In the meantime, differently from the vehicle air conditioner, a vehicular heat pump system which can selectively carry out cooling and heating by converting a refrigerant flow direction using one refrigerant cycle has been applied. For instance, the heat pump system includes two heat exchangers (one being an indoor heat exchanger mounted inside an air-conditioning case for exchanging heat with air blown to the interior of the vehicle; and the other one being an outdoor heat exchanger for exchanging heat outside the air-conditioning case), and a direction changing valve for changing a refrigerant flow direction. Therefore, according to the flow direction of the refrigerant by the direction changing valve, the indoor heat exchanger serves as a heat exchanger for cooling when the cooling mode is operated, and serves as a heat exchanger for heating when the heating mode is operated.
Various kinds of the vehicular heat pump systems have been proposed, and FIG. 1 illustrates a representative example of the vehicular heat pump system.
As shown in FIG. 1, the vehicular heat pump system includes: a compressor 30 for compressing and discharging a refrigerant; an indoor heat exchanger 32 for radiating heat of the refrigerant discharged from the compressor 30; a first expansion valve 34 and a first bypass valve 36 mounted in parallel for selectively passing the refrigerant passing through the indoor heat exchanger 32; an outdoor heat exchanger 48 for exchanging heat with the refrigerant passing through the first expansion valve 34 or the first bypass valve 36 outdoors; an evaporator 60 for evaporating the refrigerant passing through the outdoor heat exchanger 48; an accumulator 62 for dividing the refrigerant passing through the evaporator 60 into a gas-phase refrigerant and a liquid-phase refrigerant; an inside heat exchanger 50 for exchanging heat between the refrigerant supplied to the evaporator 60 and the refrigerant returning to the compressor 30; a second expansion valve 56 for selectively expanding the refrigerant supplied to the evaporator 60; and a second bypass valve 58 mounted in parallel with the second expansion valve 56 for selectively connecting an outlet side of the outdoor heat exchanger 48 and an inlet side of the accumulator 62.
In FIG. 1, the reference numeral 10 designates an air-conditioning case in which the indoor heat exchanger 32 and the evaporator 60 are embedded, the reference numeral 12 designates a temperature-adjustable door for regulating a mixed amount of cold air and warm air, and the reference numeral 20 designates a blower mounted at an inlet of the air-conditioning case.
According to the heat pump system having the above structure, when a heat pump mode (heating mode) is operated, the first bypass valve 36 and the second expansion valve 56 are closed, and the first expansion valve 34 and the second bypass valve 58 are opened. Moreover, the temperature-adjustable door 12 is operated as shown in FIG. 1. Accordingly, the refrigerant discharged from the compressor 30 passes through the indoor heat exchanger 32, the first expansion valve 34, the outdoor heat exchanger 48, a high pressure part 52 of the inside heat exchanger 50, the second bypass valve 58, the accumulator 62 and a low pressure part 54 of the inside heat exchanger 50 in order, and then, is returned to the compressor 30. That is, the indoor heat exchanger 32 serves as a heater and the outdoor heat exchanger 48 serves as an evaporator.
When an air-conditioning mode (cooling mode) is operated, the first bypass valve 36 and the second expansion valve 56 are opened, and the first expansion valve 34 and the second bypass valve 58 are closed. Furthermore, the temperature-adjustable door 12 closes a path of the indoor heat exchanger 32. Therefore, the refrigerant discharged from the compressor 30 passes through the indoor heat exchanger 32, the first bypass valve 36, the outdoor heat exchanger 48, the high pressure part 52 of the inside heat exchanger 50, the second expansion valve 56, the evaporator 60, the accumulator 62 and the low pressure part 54 of the inside heat exchanger 50 in order, and then, is returned to the compressor 30. That is, the indoor heat exchanger 32 closed by the temperature-adjustable door 12 serves as a heater in the same with the heat pump mode.
However, in the case of the conventional vehicular heat pump system, in the heat pump mode, the indoor heat exchanger 32 mounted inside the air-conditioning case 10 serves as a heater to carry out heating and the outdoor heat exchanger 48 mounted outside the air-conditioning case 10, namely, in front of the engine room of the vehicle, serves as an evaporator to exchange heat with outdoor air.
In this instance, when temperature of the surface of the outdoor heat exchanger 48 falls below the freezing point while the refrigerant flows induced into the outdoor heat exchanger 48 exchanges heat with outdoor air, frosting is formed on the surface of the outdoor heat exchanger 48.
When frosting on the surface of the outdoor heat exchanger 48 keeps expansion, because the outdoor heat exchanger 48 cannot absorb heat, temperature and pressure of the refrigerant inside the system lower and temperature of the air discharged from the interior of the vehicle also lowers so as to remarkably reduce heating performance of the system, and a liquid-phase refrigerant may be induced into the compressor so as to deteriorate stability of the system.
Therefore, the conventional vehicular heat pump system stops operation of the heat pump system when frosting is formed on the surface of the outdoor heat exchanger 48, and then, controls to reoperate the system when frosting is removed. As described above, because the operation of the heat pump system is stopped when frosting is generated, it deteriorates heating performance. In this instance, when an electric heater is operated for heating, consumption of electric power increases and it causes decrease in mileage of electric vehicles or hybrid vehicles.
In order to solve the above-mentioned problems, Korean Patent No. 1342931 discloses ‘heat pump system for vehicle’ which has been filed by the same inventor as the present invention. In Korean Patent No. 1342931, when frosting is formed on the surface of the outdoor heat exchanger, the heat pump system carries out a defrosting mode in such a way that a refrigerant bypasses the outdoor heat exchanger to recover waste heat of electronic units of the vehicle through heat supplying means, namely, a chiller, such that the heat pump system can keep heating even though frosting is formed on the surface of the outdoor heat exchanger.
However, the conventional heat pump system has a disadvantage in that indoor discharge temperature lowers about 5° C. to 10° C. because the heat pump system uses just the waste heat of the electronic units of the vehicle as a heat source when frosting is formed on the surface of the outdoor heat exchanger, and in that a PTC heater must be additionally operated in order to keep indoor temperature.
Moreover, the conventional heat pump system has another disadvantage in that a refrigerant flow rate is reduced because the refrigerant discharged from the indoor heat exchanger is not condensed sufficiently, so it causes deterioration in heating performance and cooling performance.